While the show is known for its dramatic weight loss transformations -- most winners lose more than 50% of their body weight -- Frederickson appeared extremely thin. And the looks on trainers Bob Harper and Jillian Michaels' faces could be interpreted as shock -- or dismay.

Frederickson went from 260 pounds to 105 pounds, losing 59.62% of her body weight. At 5 feet, 5 inches tall, that puts her body mass index at 17.5. Anything under 18.5 is considered by the National Institutes Of Health to be underweight.

NBC on Wednesday declined to comment on its $250,000 grand prize winner, a voice-over artist who lives in Los Angeles.

Social media, however, was buzzing.

"Are you kidding?" One woman wrote on the show's Facebook page. "Rachel looks anorexic! She has gone from one extreme to the other!"